This invention pertains to microlithography methods and apparatus employing a charged particle beam (e.g., electron beam) to perform projection-exposure of, e.g., a circuit pattern defined by a reticle or the like onto a suitable substrate (e.g., semiconductor wafer). More specifically, the invention pertains to such methods and apparatus permitting projection-exposure of desired exposure units on the reticle while achieving improved throughput and transfer accuracy.
Projection microlithography is used extensively in the manufacture of semiconductor integrated circuits, displays, and the like. Virtually all contemporary microlithography methods utilize a beam of light (typically ultraviolet light) to perform pattern transfer (microlithography using a light beam is termed xe2x80x9copticalxe2x80x9d microlithography). However, due to the current inability of light to achieve resolution of feature sizes required for the next generation of integrated circuits, various microlithography methods using a beam other than light are now being considered. Among such new methods, microlithography using a charged particle beam (e.g., an electron beam) is currently the subject of intensive investigation (microlithography using a charged particle beam is termed xe2x80x9cCPBxe2x80x9d microlithography).
In either optical or CPB microlithography, the pattern to be transferred to the substrate is defined by a reticle. All or a portion of the reticle is illuminated by an xe2x80x9cillumination beamxe2x80x9d passing through an xe2x80x9cillumination-optical systemxe2x80x9d located upstream of the reticle. Portions of the illumination beam passing through the illuminated region of the reticle the xe2x80x9cpatterned beamxe2x80x9d or xe2x80x9cimaging beamxe2x80x9d) are projected using a projection-optical system (located downstream of the reticle) onto a semiconductor wafer or other suitable substrate. The substrate is coated with a resist that, when exposed to the patterned beam, is imprintable with the pattern.
The reticle can define the pattern for a single chip (i.e., for a single xe2x80x9cdiexe2x80x9d) or for multiple dies. Alternatively, the reticle can define a single inspection pattern or multiple inspection patterns. The entire reticle need not be exposed in a single exposure or xe2x80x9cshot.xe2x80x9d For example, the reticle can comprise multiple regions (xe2x80x9cexposure unitsxe2x80x9d) that are individually exposed. To achieve such selected exposure, certain conventional optical microlithography systems have a movable mechanical field aperture (xe2x80x9creticle blindxe2x80x9d) that trims the illumination beam to illuminate only a desired portion of the reticle while not illuminating other portions of the reticle. Certain other conventional optical microlithography apparatus perform exposure by scanning selected exposure fields of the reticle using a slit-shaped portion of the illumination beam. In either method of exposure, the illumination beam is maintained at a constant intensity and focal position for the selected exposure field. I.e., within the selected exposure field, the beam intensity and focal position are not variable.
In conventional optical microlithography apparatus employing a reticle blind, it is desirable to have the reticle blind located immediately adjacent (just upstream of) the reticle to minimize defocusing of the image of the reticle blind on the reticle. Also, with such apparatus that perform scanning exposure of the reticle, it is necessary to scan the field aperture as the reticle stage is being scanned, as disclosed in Japanese Kxc3x4kai Patent Publication No. Hei 6-232031.
With a field aperture, the magnitude of defocusing of the image of the field aperture at the image plane is proportional to the axial distance between the field aperture and the reticle pattern. To solve this problem, the illumination-optical system in some conventional optical microlithography systems defines a plane conjugate with the reticle, and the field aperture is situated at that plane. However, such a configuration results in excessive complexity of the illumination-optical system. Furthermore, even with such a configuration, as the reticle is scanned during exposure, it is necessary also to scan the field aperture, as disclosed in Japan Kxc3x4kai Patent Publication No. Hei 7-94387.
Whereas field apertures have been employed as described above with conventional optical microlithography systems, no such employment of field apertures has been proposed for CPB microlithography apparatus and methods. For example, placing a reticle blind immediately upstream of the reticle is not practical with conventional CPB microlithography apparatus in which multiple coils, deflectors, and the like are typically situated just upstream of the reticle to minimize disturbance fields at the reticle. Also, in situations in which the field aperture should be scanned along with scanning the reticle stage, having to include the necessary scanning mechanism for the field aperture precludes placement of the field aperture adjacent to and immediately upstream of the reticle.
Many semiconductor xe2x80x9csystemxe2x80x9d integrated circuits (termed xe2x80x9csystem LSIxe2x80x9d circuits) have been produced recently. System LSI circuits combine logic circuits and memory circuits in a single chip. During manufacture, the layer-to-layer step differences within each system LSI chip tend to be relatively high, making them susceptible to differences in the best-focus position during microlithography steps. Also, during manufacture of such chips, the optimal exposure dose varies depending upon positional factors such as differences in resist thickness due to variations in step height and/or differences in beam reflectivity of material underlying the resist. Consequently, it has not been possible with conventional CPB microlithography apparatus employing single-shot and/or slit-illumination scanning exposure to control the exposure dose and focal position adequately in each exposure unit.
Furthermore, in the face of such problems, conventional CPB microlithography methods and apparatus cannot achieve sufficiently high throughput to be of practical utility in a contemporary wafer-fabrication facility, especially for mass-producing semiconductor devices such as DRAMs and the like. Low throughput with conventional CPB systems is also the result of limitations on the size of the optical field that will provide good correction of aberrations.
The present invention addresses the types of problems summarized above with the prior art. An object of the invention is to provide methods and apparatus that projection-expose a pattern, defined on a reticle or analogous device, onto a sensitive substrate using a charged particle beam (e.g., electron beam). Another object is to provide such methods and apparatus that achieve such ends with an acceptable level of throughput and pattern-transfer accuracy.
According to a first aspect of the invention, methods are provided for performing projection microlithography using a charged particle beam. According to a first representative embodiment of such a method, a pattern is defined on a reticle for transfer of the pattern to a sensitive substrate (e.g., semiconductor wafer). The pattern is divided on the reticle into separate exposure units each defining a respective portion of the pattern. The individual exposure units are sequentially illuminated using a charged-particle illumination beam. As particles in the illumination beam pass through each exposure unit, a charged-particle patterned beam is formed propagating downstream of the reticle. For each illuminated exposure unit, the respective patterned beam is projected onto a respective region of the sensitive substrate. Thus, an image of the illuminated exposure unit is formed on the respective region of the substrate. The respective regions are located so as to cause the images of the illuminated exposure units to be stitched together. Also according to the method, ON/OFF control data are provided for the illumination beam for each of the exposure units on the reticle. Each exposure unit is exposed based on the ON/OFF data.
In the foregoing method, the pattern can be defined on the reticle such that the exposure units are arranged in a two-dimensional array in X and Y directions on the reticle. The exposure units are sequentially illuminated in the X direction by a controlled deflection of the illumination beam. In coordination with illumination of the exposure units, the respective patterned beam is controllably deflected in the X direction. The X-direction deflections are coordinated with movements of the reticle and substrate in the Y direction as required to sequentially project each exposure unit onto a desired location on the sensitive substrate.
Whenever an exposure unit is encountered in which the illumination beam should be OFF, exposure is advanced to a subsequent exposure unit without illuminating the previous exposure unit.
The controlled deflections of the patterned beam in the X direction can be coordinated with continuous movements of the reticle and substrate in the Y direction as required to sequentially project each exposure unit onto a desired location on the sensitive substrate.
At a Y coordinate including one or more exposure units in which the illumination beam should be OFF, the reticle and substrate can be moved at respective velocities that are increased relative to respective base velocities at a Y coordinate having no exposure units in which the illumination beam should be OFF. The increased velocities are greater than base velocities according to a number of exposure units arrayed in the X direction at the respective Y coordinate in which the illumination beam should be OFF.
According to second representative embodiment of methods according to the invention, a pattern to be transferred to a sensitive substrate is defined on a reticle. The pattern is divided on the reticle into multiple exposure units. Individual exposure units are sequentially illuminated with an illumination charged particle beam to form, for each illuminated exposure unit, a respective patterned beam propagating downstream of the reticle. The respective patterned beams are sequentially projected to form respective images of the exposure units on the sensitive substrate. The respective images are situated on the substrate such that the images are stitched together on the sensitive substrate to form the pattern on the substrate. For each exposure unit, data are provided for at least one of exposure-dose value (xcexcC/cm2) and focal position, wherein the steps of illuminating the exposure units on the reticle and projecting the respective patterned beam are performed according to the data. By way of example, the exposure-dose value and focal-position data can pertain to a characteristic of the sensitive substrate.
According to another aspect of the invention, charged-particle-beam projection-exposure apparatus are provided. A first representative embodiment of such an apparatus comprises a movable substrate stage and a movable reticle stage on which a sensitive substrate and reticle, respectively, are mountable. The reticle defines a pattern to be projection-transferred to the substrate, wherein the pattern is divided on the reticle into multiple exposure units two-dimensionally arrayed in X and Y directions. The apparatus also includes an illumination-optical system that sequentially illuminates individual exposure units on the reticle with a charged-particle illumination beam. (Particles of the illumination beam passing through an illuminated exposure unit form a patterned beam propagating downstream of the reticle.) The apparatus also includes a projection-optical system that projects the patterned beam to form an image of the illuminated exposure unit at a desired location on the sensitive substrate. The images of the respective exposure units are situated on the substrate so as to be stitched together and form the pattern on the substrate. The apparatus also includes a controller connected to so as to controllably operate the reticle stage, the substrate stage, the illumination-optical system, and the projection-optical system. The controller is operable to perform one or more of the following: (1) cause the illumination-optical system and the projection-optical system to deflect the illumination beam and patterned beam, respectively, in the X direction so as to sequentially illuminate and project exposure units on the reticle; (2) cause the reticle stage and substrate stage to move coordinatedly in the Y direction to sequentially illuminate and project exposure units on the reticle; and (3) provide exposure data concerning ON/OFF control of illumination for each exposure unit.
The controller can be further operable to cause the illumination beam and the patterned beam to have a small-stroke deflection in the Y direction. In such an instance, the controller causes sequential exposure of the exposure units while continuously moving the reticle stage and substrate stage at least at a base velocity during exposure. The controller also increases the base velocity by a factor proportional to a number of exposure units, arrayed in the X direction at a respective Y coordinate value, that have OFF exposure data.
According to a second representative embodiment, a charged-particle-beam projection-exposure apparatus according to the invention can comprise, in addition to the reticle stage, substrate stage, illumination-optical system, and projection-optical system summarized above, a controller connected to so as to controllably operate the reticle stage, the substrate stage, the illumination-optical system, and the projection-optical system. The controller in such an embodiment: (1) causes the illumination-optical system and the projection-optical system to deflect the illumination beam and patterned beam, respectively, in the X direction so as to sequentially illuminate and project exposure units on the reticle; (2) causes the reticle stage and substrate stage to move coordinatedly in the Y direction to sequentially illuminate and project exposure units on the reticle; and (3) provides exposure data concerning at least one of exposure dose and focal position for each respective exposure unit.
By providing exposure data concerning ON/OFF control information for the charged particle beam for each exposure unit, it is possible to select which exposure units to expose and which not to expose with a resolution of exposure-unit size. Such selective exposure of exposure units can be performed using a blanking aperture provided in the illumination-optical system. Namely, whenever the illumination beam is to be ON for a particular exposure unit, the illumination beam is passed through the center of the blanking aperture. Whenever the illumination beam is to be OFF, the beam can be deflected and thus blocked by the blanking aperture.
By providing exposure data such as exposure-dose values and/or focal position for each exposure unit, exposure is possible (at a desired exposure-unit resolution) with accompanying changes in optimal exposure dose and/or best focus position within a chip.
The foregoing and additional features and advantages of the invention will be more readily apparent from the following detailed description which proceeds with reference to the accompanying drawings.